1. Field of the Invention
The present invention relates in general to a heater-built-in oxygen sensor, and more particularly to an oxygen sensor with improved operating reliability, which incorporates electric heating means for generating heat to maintain a desired operating temperature of an electrochemical cell which produces an electromotive force due to a difference in oxygen concentration between a measurement gas and a reference gas.
2. Discussion of the Prior Art
In the field of controlling the air/fuel ratio of an air-fuel mixture to be supplied to an internal combustion engine used for a motor vehicle, for example, it is known to use an oxygen sensor which utilizes an oxygen-ion conductive solid electrolyte material such as a zirconia ceramic, for detecting the oxygen concentration of exhaust gases emitted from the engine, according to the principle of an oxygen concentration cell.
Such an oxygen sensor for determining the oxygen concentration has a sensing element which includes a planar, columnar, tubular or otherwise shaped solid electrolyte body, and electrodes formed on the inner and outer surfaces of the solid electrolyte body. More specifically, the electrode on the inner surface of the solid electrolyte body is exposed to the ambient atmosphere, so as to serve as a reference electrode, while the electrode on the outer surface of the solid electrolyte body is exposed to an exhaust gas to be measured (measurement gas). An electromotive force is induced between the reference and measuring electrodes, due to a difference in oxygen concentration between the ambient atmosphere and the exhaust gas. The induced electromotive force is applied to a suitable measuring apparatus, to determine the oxygen concentration of the exhaust gas.
To assure accurate and reliable operation of the oxygen sensor even when the temperature of the exhaust gas is relatively low, a suitable heater or heating means is generally provided to heat at least the oxygen detecting portion of the sensing element on which the electrodes are disposed, so that the oxygen detecting portion is maintained at a desired elevated operating temperature. For example, an electric resistance rod heater is disposed in a bore formed in a tubular sensing element, as disclosed in laid-open Publication No. 57-142555 of unexamined Patent Application. Alternatively, an electric resistance heating element is embedded in a laminar structure of a planar sensing element, as disclosed in laid-open Publication No. 54-140145.
In the oxygen sensor as described above, the measuring electrode formed on the outer surface of the oxygen detecting distal portion of the sensing element is exposed to the measurement gas in the external space, while the reference electrode formed in the inner surface of the oxygen detecting portion communicates with a suitable reference gas passage which is formed through the sensing element, so as to extend from the proximal end portion to the oxygen detecting distal portion. This reference gas passage is open at the proximal end of the sensing element, so that the ambient air is introduced into the passage as a reference gas to which the reference electrode is exposed. Usually, the sensing element is accommodated in a suitable metallic covering member which has an air inlet through which the ambient air flows for introduction into the reference gas passage.
When the oxygen sensor of the type indicated just above is exposed to water or moisture, the sensing element may be adversely influenced by such water or moisture which is introduced into the covering member through the air inlet. For instance, the electrical insulation of the sensing element is deteriorated, causing inaccurate output signals of the sensor, or the oxygen partial pressure of the reference gas in the reference gas passage is lowered due to evaporation of the introduced water or moisture, whereby the output signals do not accurately reflect the oxygen concentration of the measurement gas. Further, the ceramic materials of the sensing element may be physically damaged due to exposure to the water or moisture.
In view of the above problem, laid-open Publication No. 62-214347 proposes providing an oxygen sensing element with exclusive oxygen pumping means for performing an oxygen pumping action to produce a reference atmosphere (having the predetermined or reference oxygen concentration) within an enclosed space formed in the sensing element. The provision of such oxygen pumping means necessarily increases the number of fabrication process steps and the constructional complexity of the sensor, which may result in increasing the rejection ratio during manufacture of the sensor. There is proposed an alternative solution the above problem, wherein the electrochemical cell having the measuring and reference electrodes is also utilized as the oxygen pumping means. That is, a pumping current is applied through a resistor to the measuring and reference electrodes. However, the provision of the resistor also increases the number of process steps of the sensing element. Where the resistor is incorporated within the sensing element, the resistor suffers from relatively low heat resistance.